It is well known that an ideal, insulated and ungrounded alternating current supply system would provide optimum protection for people and equipment against shock and damage. Touching of only one system conductor would be completely harmless. Since, however a real supply system exhibits inevitably finite resistive and capacitive leakage impedances between the individual conductors and ground, touching of even just one single system conductor may be hazardous if such leakage impedances become too small. It can be shown that the maximum current which will flow through an external conductor when it is touched to a supply system conductor equals the maximum voltage on the system conductor divided by the total leakage impedance of the power supply system. The total leakage impedance represents the parallel combination of all the leakage impedances of the complete alternating current supply system. It appears therefore desirable to monitor the total leakage impedance of the ungrounded alternating current supply system to provide continuous indications of its safety.
As is well known, the total leakage impedance of the AC power supply system can be determined with the aid of an AC measurement current which is injected (impressed) between system and ground; this AC measurement current will flow through the system conductors and generate in the system an alternating measurement voltage which is proportional to the total leakage impedance. While the measurement of the total leakage impedance is not difficult as long as no system voltage is being generated by the AC supply system, problems will however occur when the total leakage impedance must be monitored continuously, that is, even when the AC power supply system is generating system voltage. In the latter case, it is possible to use an AC measurement current at a measurement frequency differing clearly from the frequency of the voltage generated by the power supply system; this measurement frequency can be several times the frequency of the supply system. However, the measurement of total leakage impedance thus obtained is subject to serious errors because the value of the total leakage impedance at system frequency is generally substantially different from its value as measured at the measurement frequency.
The present invention has as an object the providing of a simple method and apparatus for measuring accurately the total leakage impedance of the AC power supply system at the system operating frequency even while the power supply system is turned on and operating.